Water heaters such as gas water heaters generally have a hot water preheating function for rapidly supplying hot water which matches a set temperature for hot water. There are two types of pipe structures (or flow path types) for preheating hot water. One of them is an internal circulation preheating structure, and the other is an external circulation preheating structure (recirculation structure).
Here, as shown in FIG. 1, the pipe structure of the external circulation preheating system consists of a closed circuit where a pipe (12) is connected to a water heater (11) to form a circulation path of the hot water. In addition, it is common to install multiple water faucets (13) throughout the whole closed circuit of the pipe to draw hot water through the water faucets.
In the pipe structure of the external circulation preheating system, temperature of the hot water flowing inside the pipe varies within the length of the pipe connected. This is because there are differences such as in the heat loss due to heat radiation depending on the length of the pipe, and heat transfer rate to the outside air depending on the pipe material.
For the above reasons, in the conventional external circulation preheating method, the average temperature of the hot water inside the outer pipe is measured, and in order to supply hot water of uniform temperature to the user, a temperature sensor (14) is installed at the end location of the whole closed circuit (location of the pipe farthest from the hot water outlet location of the water heater), so that the hot water preheating temperature is controlled by using the temperature sensed by the temperature sensor.
However, such a method has many disadvantages such as requiring an additional temperature sensor to be provided in the external pipe. Particularly, as the length of the external pipe becomes longer, the connection line between the water heater and the temperature sensor becomes longer, which is inconvenient for installation. In addition, in terms of system maintenance, frequent repairs are needed.
Further, there is a problem in that as the location in the external circulation pipe for which the temperature needs to be measured increases, equal number of temperature sensors need to be installed at each location, and when malfunction such as disconnection/short-circuit occurs in the temperature sensor installed in this way, there are no means to measure the temperature inside the pipe, thereby there is a problem in that it becomes impossible to control preheating of hot water.